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    <title>Chapter 2.  Access Method Configuration</title>
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          <th colspan="3" align="center">Chapter 2. 
		Access Method Configuration
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          <div>
            <h2 class="title"><a id="am_conf"></a>Chapter 2. 
		Access Method Configuration
        </h2>
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      <div class="toc">
        <p>
          <b>Table of Contents</b>
        </p>
        <dl>
          <dt>
            <span class="sect1">
              <a href="am_conf.html#am_conf_intro">
        What are the available access methods?
    </a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="am_conf.html#idp50599376">Btree</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="am_conf.html#idp50705400">Hash</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="am_conf.html#idp50708952">Heap</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="am_conf.html#idm1385000">Queue</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="am_conf.html#idp50715336">Recno</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="am_conf_select.html">Selecting an access method</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="am_conf_select.html#idp50702528">Btree or Heap?</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="am_conf_select.html#idp50755552">Hash or Btree?</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="am_conf_select.html#idp50569200">Queue or Recno?</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="am_conf_logrec.html">Logical record numbers</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="general_am_conf.html">General access method configuration</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="general_am_conf.html#am_conf_pagesize">Selecting a page size</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="general_am_conf.html#am_conf_cachesize">Selecting a cache size</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="general_am_conf.html#am_conf_byteorder">Selecting a byte order</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="general_am_conf.html#am_conf_dup">Duplicate data items</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="general_am_conf.html#am_conf_malloc">Non-local memory allocation</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="bt_conf.html">Btree access method specific configuration</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="bt_conf.html#am_conf_bt_compare">Btree comparison</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="bt_conf.html#am_conf_bt_prefix">Btree prefix comparison</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="bt_conf.html#am_conf_bt_minkey">Minimum keys per page</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="bt_conf.html#am_conf_bt_recnum">Retrieving Btree records by logical record number</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="bt_conf.html#am_conf_bt_compress">Compression</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="hash_conf.html">Hash access method specific configuration</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="hash_conf.html#am_conf_h_ffactor">Page fill factor</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="hash_conf.html#am_conf_h_hash">Specifying a database hash</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="hash_conf.html#am_conf_h_nelem">Hash table size</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="heap_conf.html">Heap access method specific configuration</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rq_conf.html">Queue and Recno access method specific configuration</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="rq_conf.html#am_conf_recno">Managing record-based databases</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rq_conf.html#am_conf_extentsize">Selecting a Queue extent size</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rq_conf.html#am_conf_re_source">Flat-text backing files</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rq_conf.html#am_conf_renumber">Logically renumbering records</a>
                </span>
              </dt>
            </dl>
          </dd>
        </dl>
      </div>
      <div class="sect1" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h2 class="title" style="clear: both"><a id="am_conf_intro"></a>
        What are the available access methods?
    </h2>
            </div>
          </div>
        </div>
        <div class="toc">
          <dl>
            <dt>
              <span class="sect2">
                <a href="am_conf.html#idp50599376">Btree</a>
              </span>
            </dt>
            <dt>
              <span class="sect2">
                <a href="am_conf.html#idp50705400">Hash</a>
              </span>
            </dt>
            <dt>
              <span class="sect2">
                <a href="am_conf.html#idp50708952">Heap</a>
              </span>
            </dt>
            <dt>
              <span class="sect2">
                <a href="am_conf.html#idm1385000">Queue</a>
              </span>
            </dt>
            <dt>
              <span class="sect2">
                <a href="am_conf.html#idp50715336">Recno</a>
              </span>
            </dt>
          </dl>
        </div>
        <p>
        Berkeley DB currently offers five access methods: Btree, Hash, Heap, Queue and Recno.
    </p>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="idp50599376"></a>Btree</h3>
              </div>
            </div>
          </div>
          <p>
            The Btree access method is an implementation of a sorted, balanced
            tree structure.  Searches, insertions, and deletions in the tree
            all take <span class="emphasis"><em>O(height)</em></span> time, where 
            <span class="emphasis"><em>height</em></span> is the number of levels in the
            Btree from the root to the leaf pages. The upper bound on the
            height is <span class="emphasis"><em>log base_b N</em></span>, where 
            <span class="emphasis"><em>base_b</em></span> is the smallest number of keys on a
            page, and <span class="emphasis"><em>N</em></span> is the total number of keys
            stored.
        </p>
          <p>
            Inserting unordered data into a Btree can result in pages that
            are only half-full. DB makes ordered (or inverse ordered)
            insertion the best case, resulting in nearly full-page space
            utilization.
        </p>
        </div>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="idp50705400"></a>Hash</h3>
              </div>
            </div>
          </div>
          <p>
            The Hash access method data structure is an implementation of
            Extended Linear Hashing, as described in "Linear Hashing: A New
            Tool for File and Table Addressing", Witold Litwin,
            <span class="emphasis"><em>Proceedings of the 6th International Conference on Very
            Large Databases (VLDB)</em></span>, 1980.
        </p>
        </div>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="idp50708952"></a>Heap</h3>
              </div>
            </div>
          </div>
          <p>
            The Heap access method stores records in a heap file. Records
            are referenced solely by the page and offset at which they are
            written. Because records are written in a heap file, compaction
            is not necessary when deleting records, which allows for more
            efficient use of space than if Btree is in use. The Heap access
            method is intended for platforms with constrained disk space, 
            especially if those systems are performing a great many record
            creation and deletions.
        </p>
        </div>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="idm1385000"></a>Queue</h3>
              </div>
            </div>
          </div>
          <p>
            The Queue access method stores fixed-length records with logical
            record numbers as keys.  It is designed for fast inserts at the
            tail and has a special cursor consume operation that deletes and
            returns a record from the head of the queue.  The Queue access
            method uses record level locking.
        </p>
        </div>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="idp50715336"></a>Recno</h3>
              </div>
            </div>
          </div>
          <p>
            The Recno access method stores both fixed and variable-length
            records with logical record numbers as keys, optionally backed by a
            flat text (byte stream) file.
        </p>
        </div>
      </div>
    </div>
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